Add ability to invoice customer for all projects in a given time period

I am a professional who sends out invoices on a monthly basis. In a particular month, I sometimes do more than one project for a particular client. I would like to be able to:

Choose a time period
Choose a customer
Invoice that customer for work done on multiple projects during that time period. This would include:

A cover page including a figure representing all time billed during the period on all projects
Separate pages for detailed invoices for each project billed
Ability to apply retainer invoice to the total time billed during the period, instead of separately to each project during the period

To give you a better idea of what I'm talking about, I am including a screenshot from the cover page of my current invoices.